Anhydrides of carboxylic acids have the general formula: ##STR1## where R and R' may be the same or different organic groups. They may be made by several processes including the reaction of carboxylic acids or their salts with strong dehydrating agents, as for example phosphorus oxychloride or anhydrides of other carboxylic acids. Polyermic anhydrides, particularly polymeric anhydrides of alpha-beta-unsaturated monocarboxylic acid polymers, may be made according to the process of Jones, U.S. Pat. No. 3,137,660, by treating polymers of the monocarboxylic acid with such strong dehydrating agents. The resulting polymers contain both acid and anhydride groups attached to the polymer backbone, and have very low decomposition temperatures.
Further, the Jones process is not suited to the preparation of copolymers from ester monomers with anhydride monomers, since the use of such comonomer mixtures as, for example, methyl methacrylate with methacrylic acids leads to insoluble (crosslinked) polymers. Polymeric anhydrides which are essentially free of acid and inorganic contaminants, and which have high thermal decomposition temperatures may be made according to the process of Kopchik, European Patent Publication No. 76691, by treating polymers containing the monocarboxylic acid groups in a reaction vessel under devolatilization conditions.
Other processes for making polymeric glutaric anhydrides by cyclizing adjacent, pendant acid groups include Tsunoda et al., Japanese Kokai 60/231756, Kato et al., Japanese Kokai 61/254608, and Otani et al., Japanese Kokai 61/43604; the last employing low levels of ammonia or an amine to catalyze the cyclization of the acid groups into anhydrides and/or imides. Sasaki et al., in Japanese Kokai 60/184505 discloses cyclization of adjacent t-butyl (meth)acrylate mers in a polymer by heating them to liberate isobutylene; in this thermal process isobutylene is liberated from non-adjacent mers as well, to leave pendant, free acid groups in the low-molecular-weight, crystalline polymer.
The above processes require the presence of the monocarboxylic acid in the precursor feedstock in order to form anhydride, or create carboxylic acid in the anhydride polymer. An object of the present invention is to provide a process which does not require such a monocarboxylic acid in the precursor feedstock, or introduce acid into the resulting polymer. A further object is to present a process which may be used to prepare amorphous copolymers of, for example, acrylic or methacrylic esters and their corresponding anhydride essentially free of monocarboxylic acid and inorganic contaminants, yet which, depending upon the degree of reaction chosen, the feedstock acrylic polymer, the type of amine used, and process conditions, may also be used to prepare such amorphous copolymers containing monocarboxylic acid and/or amide functionality, as well, starting with a feedstock polymer that contains no acid or amide functionality.
Yet another object of the present invention is to produce polymers which are soluble, non-crosslinked, and possess high glass-transition temperatures and thermal-decomposition temperatures, and to produce all-acrylic polymers possessing such properties. Other objects of the invention will be apparent from the discussion below.